Two-stage pulping process with an isothermal first stage



May 14, 1968 L. J. GORDON ET AL TWO-STAGE PULPING PROCESS WITH AN ISOTHERMAL FIRST STAGE Filed Feb.

CONTINOUS ISOTHERMAL TWO STAGE PULPING Inerr Gas or Air HEAT EXCHANGER TOP SEPERATOR STeum and air out STEAMING VESSEL i STeoml STAGE ACCUMULATOR HEAT EXCHANGER STRAINERQ P 6 l STAGE DIGESTER OUTLET DEVICE 1Q SEPERATOR HIGH PRESSURE FEEDER 2 HEAT EXCHANGER TOP SEPERATOR 2 STAGE DIGESTER T 27 2 STAGE ACCUMULATOR 22 HEAT EXCHANGER INVENTORS.

LYLE J. GORDON BERRY W. BAILEY ATTORNEY United States Patent 3,383,277 TWO-STAGE PULPING PROCESS WITH AN ISOTHERMAL FIRST STAGE Lyle J. Gordon, Everett, and Berry W. Bailey, Mukilteo, Wash., assignors to Scott Paper Company, Delaware County, Pa., a corporation of Pennsylvania Continuation-in-part of application SeLNo. 344,631, Feb. 13, 1964. This application Feb. 6, 1967, Ser. No. 632,121

4 Claims. (Cl. 162-19) ABSTRACT OF THE DISCLOSURE This invention is directed to an improved twostage sulfite wood-pulping process in which the first-stage liquor is maintained at a relatively high constant temperature and in which the liquor liquid level is below the top of the chip pile in the first-stage digester; in this process first-stage liquor is continuously recycled through the digester and introduced at the top of the digester so that chips extending above the liquor level are continuously wetted with recycled liquor which has been introduced at the top of the digester. Also, in this process, an overpressure is continuously maintained during the operation of the first-stage digester. In order to maximize the benefits of the instant invention, the wood chips to be pulped according thereto are preferably pre-steamed prior to the pulping operation. The benefits of the instant invention are further optimized by maintaining the combined sulfur dioxide content of the first-stage liquor in the range from about 2.00 to 3.50 grams per 100 ml. of solution and the first-stage liquor pH in the range from about 6 to 8.

This application is a continuation-in-part of application Ser. No. 344,631, filed Feb. 13, 1964, now abandoned.

This invention pertains to a method for rapid pulping of wood; more particularly, this method pertains to a two-stage sulfite pulping process having an isothermal first-stage adaptable to use in a batch or continuous process and useful for softwoods as well as hardwoods.

In the manufacture of paper from wood chips, the actual pulp liberation from the chips takes up a consider able portion of the total time sequence. As is well known in the art, any reduction in pulping time and any improvements in pulp properties are desirable. As one of the criteria used in the pulping art to evaluate pulping processes bleaching is used to show the removal of the undesirable lignin component of wood. Another criterion is the rate by which lignin can be removed when put on the same bleachability basis. Other evaluating conditions are the rate by which the lignin can be removed when compared with a fast system not necessarily involving the same lignin removing chemicals, the amount of liquor used to remove the lignin from the pulps, and the amount of active material in the liquor. Further, standards may be set to obtain a particular combination of results which may not necessarily be higher than any other previously obtained results such as absolute yield or pulp quality. Some of the established operating standards represent desirable goals due to the need for utilization of certain species of wood or a mixture of wood species.

Thus, the present method involves the use of a twostage sulfite pulping process for pulping softwood and hardwood, i.e., coniferous and deciduous wood chips, whereby increased yields of pulp and a speedier pulping operation are obtained at equivalent liquor content when compared to a fast ammoniumbase, single-stage sulfite process. The fast single-stage process used represents a 3,383,277 Patented May 14, 1968 "ice heretofore generally accepted criterion for producing a given amount of pulp in a given time.

In order to compare and properly evaluate the present method, reference is made to a process disclosed in U.S. Patent No. 2,885,317. The present process also represents an improvement thereover, when compared not solely on the basis of the rate of pulp production, but also on the basis of the novel manner by which the present process may be run-either continously or batchwise.

According to the two-stage, soda-base process described in U.S. 2,885,317, the wood is sulfited in the first step with an aqueous solution containing sulfite and/or bisulfite ions and alkaline earth metal ions. The temperature is augumented continuously, or in steps, to the desired level during the sulfiting period. A high overpressure is obtained by filling the first-stage digester completely with the sulfiting liquor.

The instant process eliminates the slow rise of the temperature of the first-stage liquor required by prior art processes. Instead, wood chips pulped according to the instant invention are introduced into a first-stage digester in which the sulfite liquor is kept at a constant temperature in the range from about C. to C. This constant temperature is therefore the operative firststage pulping liquor temperature. The wood chips to be introduced into the pulping process may optionally be pre-steamed prior to first-stage pulping. In the first-stage digester the liquor level is maintained below the level of the chips therein, but at least at a level of one-half of the height of the chip pile, and first-stage liquor is continuously removed from the first-stage digester and reintroduced into the first-stage digester at a level above the top of the chip pile, in a spray form, so that chips extending above the liquid level of the first-stage liquor are continuously wetted with the first-stage liquor. Additionally, an overpressure in the range from about 5 to 100 p.s.i. above the vapor pressure of the first-stage liquor is continuously maintained in the first-stage digester during the pulping operation. I

Furthermore, pulping liquor having higher initial combined sulfite dioxide concentration, thereby enables more quick chip penetration and hemicel'lulose stabilization, by only partially filling the first-stage digester with liquor. However, it is disadvantageous to use liquor having an excessive concentration of chemicals, e.g., in excess of 3.5 gr. of combined sulfur dioxide, per 100 ml. of solution, as the excess chemicals buffer the second-stage liquor and lengthen the time required to achieve the desired result. Besides, the higher chemical concentration renders the process uneconomical. For example, with a liquorto-wood ratio of 3.5 to 1 (wood on B.D. basis), a combined sulfur dioxide concentnation of 2.5 gnams per 100 milliliters results in a suitable amount of chemical in the chips at the end of the first stage to vie sufiicient coaction with the second-stage liquor to produce the unexpected results in the present process. The pH of the first-stage liquor should be in the range from about 6 to 8.

The instant invention produces the benefit of substantially reducing residence time of wood chips in the firststage digester of a two-stage sulfite pulping process, by permitting the chips to be introduced to the first-stage digester liquor while the first-stage digester liquor is at a high operating temperature. Therefore, the timeconsuming and consequently costly prior art procedure of introducing chips to low temperature first-stage liquor and gradually heating the liquir over a period of hours to bring it to the final pulping temperature is obviated. It has heretofore been thought that the gnadual heating of the first-stage liquor to bring it to the final operating temperature was necessary in order to prevent any bur-ning of the chips which would result if the chips were subjected immediately to high temperature liquor and maintained in contact with the liquor for a prolonged period of time.

The purpose of the first stage of the two-stage sulfite pulping process is to enable the sulfite liquor to be introduced into the interstices of the wood chips, so that the second-stage of the process can produce reaction of the sulfite with the lignin in the wood chips to solubilize the lignin and bring the lignin into the solution in the second-stage liquor, whereby the cellulose fibres for use in paper making may be separated from the pulping liquor solution in which the lignin has been dissolved, by a gross separation or filtration step.

According to the instant invention the liquid level of the first-stage liquor is maintained below the chip level in the digester, but at least at a level of one-half of the height of the chips, and a substantial overpressure is also exerted in the digester. The overpressure is exerted by means of a gaseous medium which will most economically and conveniently be air. It would be expected that merely exerting an overpressure on the chips extending above the liquid level of the liquor would cause the overpressure medium, e.g. air, to be forced into the interstices of the wood chips, thereby preventing firststage liquor from readily entering these interstices to enable lignin solubilization. However, it has been found that by maintaining a continuous contact of the exposed chips, i.e. chips extending above the liquid level, with recycled first-stage liquor introduced at or above the top of the chip pile, overpressure medium will not be introduced into the interstices, but rather only first-stage liquor will be introduced.

In performing the instant process, it is desirable that residence time of the chips in the first-stage digester be minimized, because prolonged residence time may cause chip burn due to the high constant temperature of the first-stage liquor. Of course, burnt chips do not produce commercially usable wood pulp, and the efficiency of any process would be substantially diminshed if measurlable chip burn was permitted to occur.

In order to maximize the efiiciency enabled by the instant process, it is desirable that the conventional step of pre-steaming the chips be performed prior to firststage pulping of the chips. Pre-steaming is a well-known step and consists of subjecting wood chips to an atmosphere of high temperature steam. Pre-steaming can perform two valuable functions, the first of which is to expand the chips with concomitant expansion of the interstices of the chips, thereby providing larger spaces into which the liquor may flow during the pulping op eration, and thereby increasing the rate of liquor introduction into the chips. Since it is preferable to reduce residence time in the first-stage digester, to prevent chip burn and generally speed pulping, this pre-steaming operation which permits the first-stage liquor to more quickly enter the chips is quite desirable. The pre-steaming step also causes steam to enter the chip interstices and replace the air which partially fills these interstices. Again this replacement of air with steam further facilitates introduction of first-stage liquor in the pulping operation. The pre-steaming operation is carried out at a pressure in the range from about p.s.i.g. to about 25 p.s.i.g. for a time period from about one to thirty minutes. The particular time and pres-sure utilized will 'de pend upon many variables, such as the type of wood chips being pre-steamed, their moisture content, the size of the wood chips being pre steamed and the number of wood chips being pre-steamed. It is well-known in the art that excessive time and temperature of pre-steaming will cause the natural wood acids in the chips to attack the lignin in the chips, effecting an acid hydrolysis of the lignin, thereby rendering the lignin insoluble. Insolubilization of the lignin is contrary to the entire purpose of the pulping process, since it prevents the lignin from being dissolved in the liquor and thereafter filtered from the fibre residue, and cannot therefore be tolerated; therefore the skilled artisan will adjust the time and pressure (ergo, temperature) of the pro-steaming step, .according to the variables which will affect the pre-steaming operation, in order to accomplish maximum wood chip expansion and replacement of interstitial air with steam, without permitting acid hydrolysis to insolubilize the lignin. The manner of adjusting the pre-steaming conditions to optimize results without insolubilizing lignin will be readily understood by the skilled artisan. For example, data indicates that due to lignin inactivation (in solubilization) no benefit is obtained from pre-steaming carried out at 130 C. for fifteen minutes with certain types of wood chips. It has been found that pre-steaming at about C. and atmospheric pressure will generally give optimum results.

When the normal two-stage sulfite pulping methods are used, such as in the above patent, the second-stage will generally be operated at normal acid sulfite pH cooking conditions giving a maximum pulp yield that can be obtained for a particular bleachability. Increasing the combined S0 concentration in the first stage beyond the point where excess combined S0 is obtained in the second stage cannot give any additional benefits.

In the actual operation of the first constant-temperature (isothermal) stage, a temperature from C. to 180 C. may be used with C. preferred, and the desirable liquor pH is in the range from about 6 to 8. Operating at the same temperature in both stages gives a completely isothermal process. A temperature of 150 C. in the second stage appears to be optimum for shortening the cooking time when gauge pressure is 110 p.s.i. The preferred time period for wetting the chips in the first stage is from about 15 minutes to about 2 hours.

In the instant process, the sodium and ammonium-type sulfite liquors are the preferred type. Magnesium liquors may be used; however, the pH of the liquor must be adjusted to prevent its precipitation which generally occurs at about pH 5.5 and above unless a modified process is employed such as that disclosed in U.S. Patent No. 3,088,861.

Concentrations of the above sulfite liquor may be from about 2.00 to about 3.50 grams of combined SO /l00 ml. of solution with best results obtained from about 2.50 to about 3.00 grams of combined 80 /100 ml. of solution. These values are obtained analytically by the Sanders test. In order to insure the isothermal operations during the first-stage operation, the liquor is continually withdrawn during the residence time of the chips and heated to the desired temperature, which is slightly above the operating temperature. Slightly higher temperatures are needed to warm the incoming chips while they are being constantly wetted.

For purposes of the process, overpressure is intended to mean the excess pressure over the vapor pressure of the liquid in the first stage at the operating conditions. Overpressure may vary from 5 to 100 p.s.i.; usually, the variations range from about 20 to 60 psi. Conveniently, the overpressure is exerted by air, nitrogen or other inert gases. Presence of air does not detrimentally affect the process.

By employing gas for overpressure, less liquid is used in the process. Generally, a liquid-to-wood (B.D.) ratio of from about 4.0 to about 5.5 :1 is used in the process. Suitable results are obtained with a liquid-to-wood (L/W) ratio of about 4.521. The liquid level in the digester should be maintained below the top of the chip pile but above the mid-point thereof, to enable continuous wetting of the exposed chips with recycled liquor.

After the first-stage sulfiting step is completed, the liquor is separated from the chips to the extent necessary to further enable the chips to be subjected to a secondstage pulping. That is, the percent of combined sulfur remaining in the chips should not detrimentally buffer the pulping reaction in the second stage.

Pulping in the second stage is normally carried out at a pH of from about 1.0 to 2.0. Generally, the temperature of the liquor is at about the same temperature as in the first stage. However, the temperature may vary without any detrimental effect to the process.

The present process may be run either batchwise or continuously. The continuous process constitutes an aspect of this invention. Specifically, the improvement in the continuous process over the previously known processes lies in the physical and chemical isolation of the first stage from the second. This separation is achieved by having a continuous liquor-separating zone maintained above the vapor pressure of the first-stage liquor but below the overpressure in the sulfiting zone. After the sulfiting liquor has been separated, the chips are moved along by means of gas pressure to a high-pressure feeder where liquor from the second stage, and at a vapor pressure below that of feeding gas, is mixed with the chips to facilitate easy advance of the mixture to the second stage. Consequently, the process involves two liquor routing loops in the zone where the chips from the first stage are transferred to the second stagethe first loop circulated part of the withdrawn liquor is recycled from the chips back to the sulfiting stage. The major part of sulfite liquor is sent to an accumulator. Another minor part of the liquor is combined with a major amount of second-stage liquor used to advance the chips.

This second stage liquor is obtained from a fresh makeup liquor and a portion of recirculating liquor from second stage. At all times during the advancing of the chips, the pressure is higher upstream and is so maintained by the gas injection near the chip outlet of the first zone.

To understand the invention more clearly, reference is made to the figure wherein the operation of the continu ous two-stage isothermal process is depicted. Thus, chips from a bin 1 are passed through a chip meter 2, into a low pressure feeder 3. From there, the chips pass into a steaming vessel 4 where they are contacted with low pressure (about 5 p.s.i.g.) steam for about 15 minutes. Then the chips pass into a high pressure feeder 5 where a sodium base sulfite liquor at 160 C., 110 psig. and a pH of about 7 containing about 2.50 gr. combined sulfur dioxide per 100 ml. is added to the chips in a weight ratio of 3.7:1 liquor to wood (B.D. basis). Additional liquid is required for pumping the chips from the bottom of the same to separator 10. The chips fall from the top separator 10 into the continuous digester 12 where they are continuously Wetted by liquor. The chips then pass down the digester and after about one hour are removed through the outlet device 18 at the bottom of the digester.

Air is admitted to the top of digester 12 to exert an overpressure on the digester contents.

The liquid level is maintained in the digester by reinjecting spent first-stage liquor (when necessary) from the separator 19 back into the bottom of the digester.

Liquor from the top separator 10 which is not recycled for pumping the chips from the feeder is passed through heat exchanger 14 where it is brought to 160 C. The liquor is then pumped into digester 12 and sprayed over the chips.

The chips and liquor pass in parallel down the digester. Some of the liquor is removed from middle strainer 15 and reheated in heat exchanger 17 to 160 C., then returned into the center of the digester at, or slightly above, the strainer from which it was removed.

Pressure at the top of the digester is maintained at 110 p.s.i.g. by injecting an inert gas or air.

The chips removed from digester 12 are now impregnated and sulfonated to such an extent that the final sulfonation and acid hydrolysis may be carried out at low pH without lignin inactivation.

The chips removed from digester 12 by outlet device 18 are separated from the drainable liquor at separator 19. An inert gas or air may be injected at the separator to aid in liquid separation from the chips or the reduced pressure may furnish sufficient vaporization to serve this purpose.

The separated first-stage liquor in excess of that recycled to the bottom of the digester 12 or needed in the second-stage digestion is mixed with liquor circulating through heat exchanger 9 of accumulator 6 and becomes part of the makeup for fresh first-stage liquor.

The treated chips pass from the separator 19 into the high pressure feeder 21 Where they are contacted with sufficient liquor under pressure and pumped to the top separator 26 of second-stage digester 28.

The liquor used consists partly of recycle from the top separator 26 and the remainder a volume equivalent to that needed in the second stage and consisting of liquor from the second-stage accumulator 22, kept at the desired temperature by heat exchanger 24, and/or from the bottom separator 19 of the first-stage digester 12.

The second-stage accumulator is maintained at a temperature of C. and the chip suspension pumped to digester 28 will be at approximately this temperature. If a higher temperature is desired in the top of the secondstage digester, then a higher temperature may be maintained in the second-stage accumulator.

The chips fall from separator 26 into digester 28 where liquor at low pH is sprayed over them to bring the pH of the chip suspension to about 1.7. The liquor separated in top separator 26 and not used for pumping of chips, as previously described, is passed through a S0 injector 30 or mixing chamber where liquid or aqueous S0 is used to lower the pH sufficiently so that it will bring the digester contents to a pH of 1.7. Pumps in the system are labeled as 7, 8, 11, 13, 16, 20, 23, 25, 27 and 29.

To describe more amply the present process, the following examples are given as illustrative of the invention:

Example 1 COOK A:

Wood Western hemlock.

Sodium Base isothermal process. First-stage accumulator liquor:

Combined S0 2.51%.

Temperature 150 C.

Liquor/wood ratio 3.8.

Liquid/wood ratio 4.7.

First-stage cooking conditions:

Time (from start of liquor 700 Freeness 500 Freeness Mullen. 118 142 Tear..- 94 83 Tensile 10, 300 11, 200

The above example illustrates a typical operating cycle of the present process. The results are expressed in units used to measure pulp properties according to TAPPI standards disclosed herein. The isothermal conditions are maintained not only in the first stage but also in the second stage, which is generally operated at the same temperature conditions. The results in this example, when TABLE H compared to the results obtained in the ammonia base C kN F H J acid process illustrated in Example 2, Cook B, clearly o how the advantages of the present two-stage process. Process Stora Isothermal N113 Acid Ace. Liquor:

7.14 7.03 7.05 7.20 1.37 1.32 EXample 2 gomb. SO01. 2.50 2. 50 2.148 2. 20 0. 3 0.35 COOK emp., 90 0 50 1 0 7 70 Hrs, First Stage 3:00 3:00 1:00 1:00 Wood Western hemlockgr? got 5;45 5:30 3:30 3:30 5:30 5:15

.5 u p eta Ammoma acid Process TAPPIKNQ 10.3 15.7 14.1 17.3 10.0 15.8 Accumulato acid Percent Scr. Yield 47.5 50.4 40.1 50.1 43.0 44.3 H 1 32 Munch 141 134 133 132 142 142 p Te:11' 88 80 83 00 07 00 Comb1nedSO 0.83%. Tensile 11,100 11,000 11,000 10,000 11,000 11,000 Total 50 7,7 TimeMin. 23 19 2 Temperature 70 C. 1 U.S. 2,885,317. 2 500 Frceness. Liquor/wood ratio 5.2. 1D Liquid/wood ratio 6.1.

Cookmg Fondltlonsi o The above comparison illustrates the present process Maxlmum temperature 134 and its advantages over the known two-stage process and Tlme 9 temperature 4100 the acid sulfite process. Obviously, the yield, as well as Total llme 5 hrs-4 minutes the rate of reaction, shows this process to be an improve- Pfessure 105 P ment over the other two processes. Other advantages over Results! of p the Stora process are: (1) less liquor is required, and screenfid yleld 4532/ of Wood- (2) a higher chemical concentration may be used. Screenings 1.05% of screened pulp. r Bleachability 15.6 llAPPI K number. Example 5 F 500 F 1 In the table below the effect of varying concentrations roofless mm 055 on the pulping process is depicted.

Mullen 116 163 Tuslle 5 TABLE III.-CONCENTRATION OF COMBINED so. VERSUS YIELDS Example 3 Cook1 K L I Accumulator Liquor:

Illustrated below 1s the effect of first-stage pH with p H 0. 20 0.12 the isothermal process on yield and pulp properties while ,fi;gg}j{{ j fi? 5: 6 holding the other cooking variables constant. First-Stage Conditions:

Temperature, C-.. 140 140 Time, hours 3:00 3:00

isi ii ia i 'i TAPPI K N 14 8 14 0 1 eac a 11 y, o

TABLE I 4) Screened Yield, percent... 51.2 64.0

Cookl C D E F G Screenings, percent 0.94 0.59

pH (FirstStag0) 7.08 0.03 5.03 3.08 Acid ID0113135110- TAPPIKNumb 14.0 14.2 13.9 13.8 15.0

Percent Screened 49.4 47.8 45.7 42.9 45.0

Ie1cent Screenings 0.70 0.76 0.84 1.04 0.75

1175603911 801.... 17-1 1 a The above example illustrates the increased y1elds due %6' 113 117 103 1m 1m to the increased concentration of reactant in sulfiting T 500 Freeness 37 139 14 148 141 liquor. Thus, this element in the new combination shows 2 Frwwss" 101 109 124 126 127 the need for a proper liquor in coaction with the other T l 86 93 99 94 111 variables. As shown, the yield increased 2.8% while the 7ilg Frecnessn 0,000 0,800 9,800 0,100 0,700 0 Screenings were reduced. This was achieved in the present 500 Free c L 0 10.950 11.500 11.800 11.500 process by using the high combined sulfur dioxide with 1 Regular acid sulfite cook (NH: base), Western Hemlock. the low hquld'to'wood Example 6 Example 4 In the table below, the rate of pulping Western hemlock Comparison of different processes with the present and the reaction effect on pulp at different rates is process with wood from the same hemlock log. depicted.

TABLE IV Bleach re- Yield Cook Time (hrszmin) Ace. Acid, quirement of N0. Percent 80; pulp as Percent Scrcelngs, percent CI: Scr. Pulp percent of to Temp. Total Total Comb. Pulp The above example illustrates the rate of acid sulfite pulping. Further, the example shows the bleach requirements and the yield of pulp versus the amount of unuseable screenings.

Example 7 The following example depicts the isothermal pulping of alder chips and pulping of 25% alder and 75% hemlock chip mixture.

TABLE V.LOG AND CHIP ANALYSIS Wood Alder Hemlock Log:

Diameter, inches 7-18 15 20. 25 No. of rings 18-43 202 276 284 Volume weight of chip Percent B.D 50. 49 61.31 62.18 53.13 Wet Density, lbs/c 21. 2 18. 7 18. 5 22. 1 Dry Density, lbs/cu. f 10.7 11. 4 11. 5 11. 7 Chip Size Distribution, percent:

Retained on 1' Mesh 4. 2 1. 4 3. 2 2. 2 Retained on Mesh. 27.0 12. 1 21. 2 17. 0 Retained on Mesh 36.3 27.9 30. 9 34. 3 Retained on M Mesh. 26. 6 45. 7 21. 1 36. 6 Retained on g' Mesh- 4. 6 10. 0 2. 9 7. 2 Passing Mesh 1. 3 2. 9 0. 7 2. 7 Chemical Tests, percent:

Lignin 21.9 27. 0 26. 2 Ethyl Ether Extr- 0. 64 0.86 0.48 0.54 Hot Water Extr 4.05 3. 46 4. 37 3. 12

In the table below the data illustrate pulping of alder according to the isothermal process. Properties of pulp are included to show the unexpected results obtained.

TABLE VI.PULPING DATA ON ALDER WOOD [Process-Isothermal] Accumulator liquor:

pH 7.10 Combined S0 percent 2.51 Total S0 percent 2.80 Temperature, C. 160

First stage:

Maximum temperature, C. 150 Hours to temperature 0:30 Total time 1:00 Second stage: Temperature, C 150 Total time, hrs. 3:00 Combined S0 at blow, percent 0.18 TAPPI K No. 15.3 Sceened yield, percent 49.0 Screenings, percent of screened pulp 3.38 Visc., cp 1057 1% NaOH, sol. 14.24 Mullen: 500 Freeness 98 Tear: 500 Freeness 93 Tensile: 500 Freeness 9800 Min. to 500 Freeness 15 BEATER TESTS Mullen:

0 min. 59 10 rnin. 90 min. 104 30 min. 113 40 min. 117

Freeness: 0 min. 630 10 min. 530 20 min. 470 30 min. 390 40 min. 300 Tear:

0 min. 90

10 min. 20 min. 92 30 min. 90 40 min. 85 Tensile:

0 min 7,275 10 min 9,650 20 min 9,950 30 min. 10,700

40 min. 11,200

FIBER CLASSIFICATION Percent retained on:

14 mesh 28 mesh 48 mesh 40.48 mesh 23.92

Percent thru 8.44

Cook 1 Cook 2 Accumulator Liquor:

pH 7. 12 7. 15 Combined S01, perc 2. 50 2. 49 Total 802. percent. 8. 08 2.84 Temperature, C 160 160 First Stage:

Maximum Temperature, C 150 150 Hours to Temperature 0:30 0:30 Total Hours 1:00 1:00 Second Stage: Temperature, C... 150 150 Total Time 3:00 3:15 Combined S0 at Blow, percent 0. 19 0.11 TAPPI K No 16. 2 12.0 Screened Yield, percent 49. 3 48. 43 Screenings, percent of Screened Pulp 1. 71 1.30 Vise, cp 885 1% NaOH 01.... 17 42 16.55 Mullen:

74 89 124 131 Tear:

700 Freeness 98 99 500 Freeness 81 82 Tensile:

700 Freeness 8,100 8,900 500 Freeness 10, 600 12,000 Min. to 500 Freeness 14 24 BEATER TESTS Cook 0 min. 10 min. 20 min. 30 min. 40 min.

Mullen 1 70 116 134 140 2 78 128 144 Freeness 1 710 550 380 250 2 710 060 550 400 270 Tear 1 100 83 78 73 Cook 0min. 10 min. 20 min. 30min. 40 min. Sp. Vol. 10 min.

Tensile 1 7, 900 10. 400 11,000 11,500 1, 23 2 7, 990 10, 350 1, 700 12, 11, 900 1. 25

FIBER CLASSIFICATION Percent Retained on Mesh- Percent Cook Thru 14 28 48 100 The results in Example 7 above indicate that hardwoods, such as alder, may be pulped with advantageous results when using isothermal conditions in a two-stage sulfite pulping. However, data indicate that the pH of first-stage liquor should be about 8, or, if it is of from about 6 to 7.5, then it should be maintained at that point by injecting, additional base to keep the pH at that level. Other data indicate that increase of yields with hardwoods range up to about 2% when compared with a conventional process, such as Stora, or about 4% to 5% when compared with a normal acid sulfite process.

In ascertaining the results, the following tests were used (as indicated in the respective examples):

pH of the cooled liquid was measured at about 25 C. with a conventional pH meter.

Combined S0 concentration was obtained by Sanders test carried out as described below:

Pipette 2 mls. of solution to be tested into a 250 ml. Erlenmeyer flask containing 50 mls. of distilled water; add 5 drops of Bromophenol blue indicator; titra'te to lavender-blue end-point with N NaOH; record the number of mls. used as titration A; add 20 mls. of

saturated mercuric chloride solution; again titrate to a lavenderblue end-point with N NaOH; record the /16 -N NaOH used for this second titration as titration B. Calculate: Percent combined SO =0.1 (BA).

The screened pulp yield was expressed as the amount of pulp obtained (as a percent of the bone-dry wood) which will pass through a fiat screen with slits 0.008 in width and it represents the useable fiber.

The screenings were expressed as percentage of screened pulp.

B.D. represents bone-dry wood and, when so used, indicates the percentage of original material left, after drying, in a 105 C. oven until constant weight is obtained.

TAPPI K numbers represent bleachability. The test was carried out according to TAPPI standard 214m-50. TAPPI K numbers are a measure of the lignin content of the pulp and are indicative of bleaching chemical required.

Mullen, tear and tensile values were obtained according to the latest TAPPI standard T220m-60.

1% NaOH solubility was carried out according to TAPPI standard T212m-54. The solubility indicates the amount of short-chain carbohydrate material present.

Freeness was obtained according to the method given in TA-PPI standard T227m-58.

Viscosity, expressed in centipoises, was measured according to the method described in TAPPI standard T230sm-50.

What is claimed is:

1. A two-stage pulping process comprising the steps of:

(1) introducing wood chips into a first-stage chip digester and immediately contacting said wood chips with an isothermal liquor selected from the group consisting of sodium-, magnesiumand ammoniumbase sulfite liquors at a temperature in the range from about 130 C. to about 180 C., having a pH in the range from about 6 to about 8 and a combined sulfur dioxide content not in excess of about 3.5 grams per ml. of solution, for a period of time sufficient to impregnate said chips with first-stage liquor Without burning any chips;

(2) maintaining the liquor level in the first-stage digester at less than the height of the column of chips in said first-stage digester and more than one-half the height of said column;

(3) continuously contacting said liquor with the chips above the liquor level by recycling liquor in said first-stage digester and to continuously wet said chips in said first-stage digester with isothermal liquor, while exerting an over-pressure in the range from about p.s.i. to about 100 p.s.i. above the vapor pressure of the first-stage liquor during said firststage pulping operation;

(4) separating the first-stage liquor from the chips;

(5) introducing said chips into a second-stage digester and immersing said chips in second-stage liquor having a pH in the range from about 1.0 to about 2.0 and a temperature in the range from about 140 C. to about 180 C.; and

(6) continuing pulping in said second-stage digester until a wood pulp is obtained.

2. A process as set forth in claim 1 wherein the weight ratio of first-stage liquor to wood chips in the first-stage digester is in the range from about 4:1 to about 5.5 :1 on

a bone dry basis.

3. A process as set forth in claim 1 including the initial steps of expanding said chips and impregnating said chips with steam by pre-steaming said chips at a pressure in the range from about 0 p.s.i.g. to about 25 p.s.i.g. for a period from about 1 minute to about 30 minutes without insolubilizing the lignin in said chips.

4. A two-stage sulfite pulping process comprising the steps of:

(1) expanding wood chips and introducing steam into said chips by pre-steaming said chips at a pressure in the range from about 0 p.s.i.g. to about 25 p.s.i.g. for a period in the range from about 1 minute to about 30 minutes without insolubilizing any substantial amounts of the lignin in said chips;

(2) promptly introducing said pro-steamed chips into a first-stage digester and immediately contacting said pre-steamed chips with an isothermal liquor selected from the group consisting of sodium-, magnesiumand ammonium-base sulfite liquors at a temperature in the range from about C. to about 180 C. having a pH in the range from about 6 to 8 and a combined sulfur dioxide content in the range from about 2.0 to about 3.5 grams per 100 ml. of solution;

(3) continually removing liquor from said first-stage digester, heating said liquor to substantially maintain the isothermal condition of said liquor, maintaining the liquid level in said first-stage digester substantially below the level of the chips in said digester but in excess of one-half of said level of chips, and continuously contacting said liquor with the chips above the liquor level 'by reintroducing said liquor into said first-stage digester to continuously wet the same;

I (4) wetting said chips with said isothermal liquor for a period in the range from about 15 minutes to about 2 hours until all of said chips are substantially impregnated with first-stage liquor without burning any chips, while maintaining an over-pressure in the range from about 5 p.s.i.g. to about 100 p.s.i.g. above the vapor pressure of said liquor; (5) separating said impregnated chips from said liquor; (6) introducing said impregnated chips into a secondstage digester filled with liquor at a temperature in the range from about C. to about C. and

a pH in the range from about 1.0 to about 2.0; and (7) continuing pulping in said second-stage digester until pulping of said chips is complete.

References Cited UNITED STATES PATENTS 2,885,317 5/1959 Collin et a1 l62-84 X 3,041,232 6/1962 Richter et al 162--17 3,078,208 2/ 1963 Cederquist et a1. 162-84 X 3,210,237 10/1965 Madison 162-84 X FOREIGN PATENTS 598,206 5/1960 Canada.

S. LEON BASHORE, Primary Examiner.

R. D. BAJEFSKY, Assistant Examiner. 

